The invention relates to a hybrid circuit and in particular, a hybrid circuit, surface mountable on a circuit board, and having a heat dissipation system.
Hybrid or sandwich circuits are electronic subassemblies in which planar conductive track structures and passive components, in particular resistors, are applied to a ceramic substrate by means of specific film technologies, with thee planar structures on the common ceramic substrate being adapted to be connected or hybridized with other components, for example ICs or relays. These further components usually are adapted to be surface mounted, i.e. using the known SMD (Surface Mounted Devices) technology, they are placed on connecting pads on the hybrid circuit and soldered there, mostly by reflow soldering.
Hybrid circuits on ceramic carrier material permit high power levels on the smallest possible constructional space. This is often accompanied by strong heating of the assembly. This strong heating must be removed or dissipated from the component. Usually, part of the dissipation takes place via the component surface and another part via the terminal pins. If the heat cannot be discharged sufficiently via the terminal pins, or if the heat exceeds the melting temperature of the solder used for soldering the pins to the circuit board, the hybrid circuit unsolders itself in the circuit board. If the individual solder points melt and become pasty, the electrical connection indeed is still maintained via the individual terminal pins, but the mechanical stability of the hybrid circuit in relation to the circuit board is no longer present.
This problem is of particular significance with SMD applications in which the hybrid circuit in its entirety is to be SMD-suitable. For obtaining high integration density, hybrid circuits nowadays are soldered to circuit boards mostly in the form of a single-in-line subassemblies. To make use of the space present in the third dimension, they are thus mounted perpendicularly to the circuit board or substrate with only one row of terminals. The above-outlined stability problem thus does not make itself felt particularly in the case of single-in-line hybrid assemblies mounted to a circuit board by through-mounting, but is significant in the case of such assemblies mounted by SMD technology. For limiting the temperature increase of the terminal pins, additional heat sinks are mounted to the hybrid circuit; or, the power level in the component is subjected to stronger limitations.
The present invention provides a hybrid circuit having an SMD-suitable terminal pin arrangement for attachment of the hybrid circuit to a circuit board. The terminal arrangement includes a plurality of terminal pins for soldering the hybrid circuit to a circuit board, in which at least part of the heat developed on the basis of the power consumption of the hybrid circuit can be dissipated via the terminal pins to the circuit board.
It is an object of the invention to provide a hybrid circuit, surface mountable on a circuit board, which has an improved system for heat dissipation and temperature limitation without the use of additional heat sinks on the hybrid circuit.
With a hybrid circuit of the present invention, this object is met in that at least one terminal pin is provided with a thermal transition resistance to the hybrid circuit which is higher than that of the other terminal pins in such a way that this terminal pin does not reach the melting temperature of the solder used at a given power consumption.
The invention will be elucidated in more detail hereinafter by way of the description of a preferred embodiment with reference to the sole drawing FIGURE.